1. Field of the Invention
The present invention relates to a holographic recording medium.
2. Description of the Related Art
A holographic memory that records data in the form of a hologram is capable of high capacity recording and, thus, attracts attention as a next-generation recording medium. There has been known a photosensitive composition for the holographic recording represented by Omnidex (registered trademark, DuPont Company) that contains, as main components, a radical-polymerizable monomer, a thermoplastic binder resin, a photo-radical polymerization initiator and a sensitizing dye. Such a photosensitive composition for the holographic recording is formed into a film and, then, the film is exposed to light beams so as to bring about interference between the light beams, thereby recording data. In regions where the light beams are applied strongly, radical polymerization occurs. When the radical polymerization proceeds, the radical-polymerizable monomers are diffused from regions where the light beams are applied weakly toward the regions where the light beams are applied strongly, resulting in a concentration gradient of the radical-polymerizable monomers. Thus, in accordance with intensities of interfered light, differences in densities of the radical-polymerizable monomers are generated and differences in refractive indexes are produced.
Recently, holographic recording media using an improved polymer matrix have been proposed. For example, Japanese Patent Application Kokai Publication No. 11-252303 discloses a holographic recording medium including a three-dimensional cross-linked polymer matrix in which polymerizable monomers are dispersed. Also, a document reports a holographic recording medium including an epoxy resin matrix in which photo-polymerizable monomers are dispersed (see T. J. Trentler, J. E. Boid and V. L. Colvin, “Epoxy-Photopolymer Composites: Thick Recording Media for Holographic Data storage”: Proceedings of SPIE, 2001, Vol. 4296, pp. 259-266).
On the other hand, U.S. Pat. No. 6,780,546 discloses a holographic recording medium containing a photo-radical polymerization initiator that exhibits high sensitivity to a blue laser beam.
However, as a result of extensive research, the present inventors have found that the sensitivity of the highly-sensitive photo-radical polymerization initiator is markedly degraded with time, resulting in poor storage stability.